Add CreateNewProcessGroup to ProcessStartInfo

[jozkee]

Will fix #44944, provided that API review approves it.

[Copilot]

Pull Request Overview

This PR adds support for creating a new process group via a CreateNewProcessGroup property on ProcessStartInfo. It extends the Windows implementation, stubs the API on Unix with proper exceptions, and includes interop constants plus cross-platform tests.

• Introduces a new CreateNewProcessGroup property in ProcessStartInfo (Windows) and a stub on Unix.
• Updates the Windows StartWithCreateProcess path to include the CREATE_NEW_PROCESS_GROUP flag and adds the corresponding interop constant.
• Adds tests to verify the property behavior on both platforms and a signal‐handling scenario using remote executor.

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/libraries/System.Diagnostics.Process/tests/*.csproj	Linked additional interop files: BOOL, SetConsoleCtrlHandler, Unix Libraries, and PosixSignal helpers
src/libraries/System.Diagnostics.Process/tests/ProcessTests.cs	Added TestCreateNewProcessGroup_HandlerReceivesExpectedSignal to exercise signal dispatch
src/libraries/System.Diagnostics.Process/tests/ProcessTests.Windows.cs	Implemented SendSignal and ReEnableCtrlCHandlerIfNeeded for Windows
src/libraries/System.Diagnostics.Process/tests/ProcessTests.Unix.cs	Implemented SendSignal stub and no-op ReEnableCtrlCHandlerIfNeeded for Unix
src/libraries/System.Diagnostics.Process/tests/ProcessStartInfoTests.cs	Added unit tests for CreateNewProcessGroup on Windows and Unix
src/libraries/System.Diagnostics.Process/tests/Interop.cs	Added GenerateConsoleCtrlEvent DllImport
src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessStartInfo.Windows.cs	Added CreateNewProcessGroup property
src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessStartInfo.Unix.cs	Added stub CreateNewProcessGroup with PlatformNotSupportedException
src/libraries/System.Diagnostics.Process/src/System/Diagnostics/Process.Windows.cs	ORs CREATE_NEW_PROCESS_GROUP into the creation flags
src/libraries/Common/src/Interop/Windows/Advapi32/Interop.ProcessOptions.cs	Added CREATE_NEW_PROCESS_GROUP constant

[jozkee]

RemoteExecutor timeouts on CI (1, 2), presumably GenerateConsoleCtrlEvent fails silently due to weridness on how tests are run. A somewhat related case is described in System.Console manual tests.
@adamsitnik, @ericstj, any thoughts?

I think preventing the test from running on CI but keep running locally may be good enough for Windows.

[jozkee]

Error narrowed to only the Windows docker image, I suspect is because is running without -t (tty/console). I tested locally with docker and was able to observe a similar behavior.

https://helixr1107v0xdcypoyl9e7f.blob.core.windows.net/dotnet-runtime-refs-pull-117105-merge-4803f8e0334a41609c/System.Diagnostics.Process.Tests/1/console.e6877ea7.log

@dotnet/runtime-infrastructure can you please confirm and take a look?

[adamsitnik]

The product changes looks good to me, I had some questions regarding the tests, but mostly to get a better understanding. Big thanks for taking the extra effort to implement the tests for the new feature! 👍